…out an authority before parsing (#13859)

**This is a backport of PR #13858 as merged into master
(ce960dd).**

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## What do these changes do?

Follow-up to #13855 for aio-libs/yarl#1937. With that change, yarl's
default WHATWG mode reads a host from `http:host/p`, `http:/host/p` and
`http:///host/p`, as browsers do. The Python and Cython request parsers
passed an absolute-form request target to `URL()` and rejected it only
if yarl found no host. They now first check that the target has `//`
followed by a non-empty authority, so `GET http:///protected` and `GET
http:/protected` stay rejected as RFC 9110 requires, whatever yarl
version is installed.

The client tests used `http:///example.com` and `http:///path` as URLs
without a host. They now use `http://` where an invalid URL is needed,
and `http:///example.com` is dropped from the invalid URL test data.
With yarl#1937 it is `http://example.com/`, and as a redirect it leads
to `example.com`, as in browsers.

## Are there changes in behavior for the user?

No with the released yarl, where such targets were already rejected.
With yarl#1937, the server keeps rejecting absolute-form targets without
an authority instead of reading a host from them.

## Is it a substantial burden for the maintainers to support this?

No, it is one string check shared by both parsers.

## Related issue number

Follow-up to #13855; needed by aio-libs/yarl#1937.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [ ] Documentation reflects the changes: N/A, no documented behavior
changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`: N/A, already listed
- [x] Add a new news fragment into the `CHANGES/` folder

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Tests with the Cython extensions built (llhttp generated, `make
cythonize`, `build_ext --inplace`): `pytest tests -n auto`, 5246 passed,
22 skipped, 17 xfailed, both with yarl master (f761c37) and with the
yarl#1937 branch (8d0f111). Pure-Python run of `test_http_parser.py`,
`test_client_functional.py` and `test_client_session.py` with the
yarl#1937 branch: 884 passed.
Lint: black, isort, codespell passed. The local pre-commit flake8 hook
fails to load `flake8-requirements` (`pkg_resources` missing), so flake8
(E, F, W) was run directly and passed. yesqa was skipped for the same
reason, because it wrongly strips `# noqa: I900`.
</details>

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

) (#13860)

<!-- Thank you for your contribution! -->

Follow-up to #13855 for aio-libs/yarl#1937. With that change, yarl's
default WHATWG mode reads a host from `http:host/p`, `http:/host/p` and
`http:///host/p`, as browsers do. The Python and Cython request parsers
passed an absolute-form request target to `URL()` and rejected it only
if yarl found no host. They now first check that the target has `//`
followed by a non-empty authority, so `GET http:///protected` and `GET
http:/protected` stay rejected as RFC 9110 requires, whatever yarl
version is installed.

The client tests used `http:///example.com` and `http:///path` as URLs
without a host. They now use `http://` where an invalid URL is needed,
and `http:///example.com` is dropped from the invalid URL test data.
With yarl#1937 it is `http://example.com/`, and as a redirect it leads
to `example.com`, as in browsers.

No with the released yarl, where such targets were already rejected.
With yarl#1937, the server keeps rejecting absolute-form targets without
an authority instead of reading a host from them.

No, it is one string check shared by both parsers.

Follow-up to #13855; needed by aio-libs/yarl#1937.

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [ ] Documentation reflects the changes: N/A, no documented behavior
changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`: N/A, already listed
- [x] Add a new news fragment into the `CHANGES/` folder

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Tests with the Cython extensions built (llhttp generated, `make
cythonize`, `build_ext --inplace`): `pytest tests -n auto`, 5246 passed,
22 skipped, 17 xfailed, both with yarl master (f761c37) and with the
yarl#1937 branch (8d0f111). Pure-Python run of `test_http_parser.py`,
`test_client_functional.py` and `test_client_session.py` with the
yarl#1937 branch: 884 passed.
Lint: black, isort, codespell passed. The local pre-commit flake8 hook
fails to load `flake8-requirements` (`pkg_resources` missing), so flake8
(E, F, W) was run directly and passed. yesqa was skipped for the same
reason, because it wrongly strips `# noqa: I900`.
</details>

(cherry picked from commit ce960dd)

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## What do these changes do?

<!-- Please give a short brief about these changes. -->

## Are there changes in behavior for the user?

<!-- Outline any notable behaviour for the end users. -->

## Is it a substantial burden for the maintainers to support this?

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obvious that would complicate the ongoing development of this project?

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## Related issue number

<!-- Will this resolve any open issues? -->
<!-- Remember to prefix with 'Fixes' if it closes an issue (e.g. 'Fixes
#123'). -->

## Checklist

- [ ] I think the code is well written
- [ ] Unit tests for the changes exist
- [ ] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`
  * The format is &lt;Name&gt; &lt;Surname&gt;.
  * Please keep alphabetical order, the file is sorted by names.
- [ ] Add a new news fragment into the `CHANGES/` folder
  * name it `<issue_or_pr_num>.<type>.rst` (e.g. `588.bugfix.rst`)
  * if you don't have an issue number, change it to the pull request
    number after creating the PR
    * `.bugfix`: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
    * `.feature`: A new behavior, public APIs. That sort of stuff.
    * `.deprecation`: A declaration of future API removals and breaking
      changes in behavior.
    * `.breaking`: When something public is removed in a breaking way.
      Could be deprecated in an earlier release.
    * `.doc`: Notable updates to the documentation structure or build
      process.
    * `.packaging`: Notes for downstreams about unobvious side effects
      and tooling. Changes in the test invocation considerations and
      runtime assumptions.
    * `.contrib`: Stuff that affects the contributor experience. e.g.
      Running tests, building the docs, setting up the development
      environment.
    * `.misc`: Changes that are hard to assign to any of the above
      categories.
  * Make sure to use full sentences with correct case and punctuation,
    for example:
    ```rst
    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.
    ```

    Use the past tense or the present tense a non-imperative mood,
    referring to what's changed compared to the last released version
    of this project.

<!-- Thank you for your contribution! -->

## What do these changes do?

In the default WHATWG mode, `http:example.com/`, `http:/example.com/`
and `http:///example.com/` now parse as `http://example.com/`, as the
WHATWG URL Standard does for the `http`, `https`, `ws`, `wss` and `ftp`
schemes (the "special authority ignore slashes" state). Previously the
text stayed the path of a URL without an authority and printed as
`http:///example.com/`. Userinfo or a port without a host
(`http:@/www.example.com`) is rejected, as for `http://@/x`. `http:`,
`http:/`, `http:?q`, `http://` and `http:///` are kept as they were,
since there is no host to read. An input with `//` is absolute against
any base, as in WHATWG, so `join()` no longer keeps the base host for
`http:///example.com/`.

Against a base with the same scheme, WHATWG resolves the same input as a
relative reference (`http:foo.com` against `http://example.org/foo/bar`
is `http://example.org/foo/foo.com`). `URL.join()` only sees the parsed
reference, so a URL parsed this way keeps its original path in the cache
(and in its pickle state), and `join()` uses it when the schemes match.
RFC 3986 mode is unchanged: it already keeps these URLs without an
authority.

In `tools/conformance/REPORT.md`, 33 rows move from "WHATWG mode differs
from WHATWG, RFC mode follows RFC 3986" to "yarl follows each standard
in its mode" (28 to 61). Four inputs now fail in WHATWG mode without
changing section. Three are `http:\\host\` (a backslash in the authority
is rejected; backslash handling is a separate change). The fourth is
`http:[61:27]/:foo` against an http base, which WHATWG only accepts
relative to the base, while yarl parses the reference alone first. "yarl
differs, RFC 3986 and WHATWG agree" is unchanged.

## Are there changes in behavior for the user?

Yes. In WHATWG mode, `URL("http:example.com/")` has host `example.com`,
and `URL("http:@/x")` raises `ValueError`.
`URL(base).join(URL("http:g"))` for an http base gives the same result
as before.

## Is it a substantial burden for the maintainers to support this?

No. The conversion is one helper called from both parsers, and join()
checks one cache key for references with an authority.

## Related issue number

Follow-up to #1931 and #1936. The "Aiohttp tests" job needs
aio-libs/aiohttp#13855 (merged) and aio-libs/aiohttp#13858, which keeps
aiohttp's server rejecting absolute-form request targets without an
authority.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`
- [x] Add a new news fragment into the `CHANGES/` folder

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Tests: `pytest tests` 2699 passed, 57 xfailed; new module
`tests/test_url_special_authority.py` fully covered.
Lint: `pre-commit` (ruff, flake8, mypy) passed.
Docs: `make doc-spelling` and `make -C docs doctest` passed.
Conformance: `python tools/conformance/compare.py --check` passes
against this tree.
</details>